1. Field of the Invention
The present invention relates to optical amplifiers, and more particularly, to a polarization-independent semiconductor optical amplifier obtained by growing on the same substrate two individually addressable angled-stripe strained layer structures with different TE-TM characteristics.
2. Description of the Prior Art
In optical communication systems, amplifiers are often used to boost or distribute optical signals in fibers. Because the fiber can support any optical polarization, it is desirable that the gain of the optical amplifier be polarization-independent. A major problem with such semiconductor optical amplifiers is an undesirable difference in optical gain for different polarizations. In other words, a semiconductor optical amplifier typically has a difference of several dB in gain between TE and TM (transverse electric and transverse magnetic) input waves. This can result in output distortion which impairs system performance. This is particularly important for analog signals for which the TE/TM gains are expected to be within a fraction of a dB.
It is well known that any optical polarization can be viewed as a linear combination of two independent polarizations, such as TE and TM waves, with a phase difference between them which may vary with time. Thus, an optical amplifier that has equal gain for the two independent polarizations will have equal gain for all polarizations.
It is also known that compressive strain in a quantum well active layer results in enhanced TE polarization, and that tensile strain results in enhanced TM polarization. These two gains have different spectral characteristics. Thus, without special care, the gain spectra of TE and TM are not the same over the full spectrum of a semiconductor optical amplifier.
FIGS. 1(a)-1(c) show the gain spectra for prior art multiquantum well structures of semiconductor lasers and amplifiers (MQW-SLAs) around 1300 nm. This data is taken from Thijs et al. in IEEE J. Quant. Electron., Vol. 30, No. 2, pp. 477-498 (February 1994). FIG. 1(a) illustrates the polarization-resolved gain spectra at 100 mA drive current of a MQW-SLA with four compressively strained wells (4C) and two tensile strained wells (2T). Similarly, FIGS. 1(b) and 1(c) show the polarization-resolved gain spectra at 100 mA drive current of a MQW-SLA with four compressively strained wells (4C) and three tensile strained wells (3T) and four tensile strained wells (4T), respectively. FIG. 1(a) has two layers in tension and four layers in compression and shows TE to have about a 4 dB gain larger than TM. On the other hand, FIG. 1(c) has four layers in tension and four layers in compression and shows TM to be larger than TE by about 5 dB. In FIG. 1(b), for three layers in tension and four layers in compression, the TE and TM curves differ by about 2 dB. The wells in tensile strain (T) have 1% strain and a thickness of 110 .ANG., while the wells in compression (C) have 1% strain and a thickness of 45 .ANG.. Such differences in gain for TE and TM are unacceptably large for current applications.
U.S. Pat. No. 5,151,818, entitled "Semiconductor Optical Amplifier" and issued to Thijs et al. describes a polarization-independent semiconductor optical amplifier that has two active layer portions in series. One portion is under tensile stress, and the other portion is under compressive stress. Polarization-independence is claimed to be achieved by adjusting the current through each portion. However, this is not satisfactory because the TE component of the signal is blocked by the TM section, and vice versa. This limits the amount of gain adjustment that can be realized. Moreover, as shown in FIG. 1(d), the TE and TM curves merely intersect at one point A, thus giving polarization-independent gain B at one wavelength C, not throughout a range of wavelengths. This intersection point is difficult to reproduce reliably and varies from device to device, limiting the usefulness of the device.
It is desired to combine the characteristics of compressive strain and tensile strain in quantum wells to create a semiconductor optical amplifier which is polarization-independent over a wide range of wavelengths. The present invention has been developed for this purpose.